1. Field of the Invention
This invention relates to solid-state lamps with improved emission characteristics. In particular, although not exclusively, embodiments of the invention concern LED-based (Light Emitting Diode) lamps with an omnidirectional emission pattern and light diffusive covers therefor.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photoluminescence materials, which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color.
Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
Typically in white LEDs the phosphor material is mixed with a light transmissive material such as a silicone or epoxy material and the mixture applied to the light emitting surface of the LED die. It is also known to provide the phosphor material as a layer on, or incorporate the phosphor material within, an optical component (a photoluminescence wavelength conversion component) that is located remotely to the LED die. Advantages of a remotely located wavelength conversion component include reduced likelihood of thermal degradation of the phosphor material and a more consistent color of generated light.
FIG. 1 shows perspective and cross sectional views of a known LED-based lamp (light bulb) 10 utilizing a remote wavelength conversion component. The lamp comprises a generally conical shaped thermally conductive body 12 that includes a plurality of latitudinal heat radiating fins (veins) 14 circumferentially spaced around the outer curved surface of the body 10 to aid in the dissipation of heat. The lamp 10 further comprises a connector cap (Edison screw lamp base) 16 enabling the lamp to be directly connected to a power supply using a standard electrical lighting screw socket. The connector cap 16 is mounted to the truncated apex of the body 12. The lamp 10 further comprises one or more blue light emitting LEDs 18 mounted in thermal communication with the base of the body 12. In order to generate white light the lamp 10 further comprises a phosphor wavelength conversion component 20 mounted to the base of the body and configured to enclose the LED(s) 18. As indicated in FIG. 1 the wavelength conversion component 20 can be a generally dome shaped shell and includes one or more phosphor materials to provide wavelength conversion of blue light generated by the LED(s). For aesthetic considerations the lamp can further comprise a light transmissive envelope 22 which encloses the wavelength conversion component.
Traditional incandescent light bulbs are inefficient and have life time issues. LED-based technology is moving to replace traditional bulbs and even CFL with a more efficient and longer life lighting solution. However the known LED-based lamps typically have difficulty matching the functionality and form factor of incandescent bulbs. In particular known LED-based lamps do not meet the required emission characteristics. Embodiments of the invention at least in-part address the limitations of the known LED-based lamps.